I don't think it that close.
6 men with 2 elite bazooka +4 SVT is more cost efficient than 5 men with shreck 3 k98.
You don't said "the most cost efficient allround unit", you said "the most cost efficient AT unit". PFs have higher AT alpha damage and higher AT DPS too if you factor in their higher penetration. In addition their vet1 ability to spot with flares comes in very handy. Assault Guards wil be better at AI to make up for that. Both units are pretty cost efficient overall.